A superabsorbent polymer (SAP) is a type of synthetic polymeric material capable of absorbing moisture from 500 to 1000 times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, or the like.
As a preparation process for such superabsorbent polymers, a process performed by reverse phase suspension polymerization or a process performed by solution polymerization has been known. For example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, and S57-198714 disclose the reverse phase suspension polymerization.
The process performed by the solution polymerization further includes a thermal polymerization method in which a polymerization gel is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays onto a belt to be polymerized and dried at the same time.
The water-containing gel polymers thus obtained through the polymerization reaction are generally marketed in a powdery form after drying and pulverization processes.
In the products made of superabsorbent polymers, permeability is an index of determining fluidity of a liquid to be absorbed. Permeability may differ depending on the properties such as particle size distribution of crosslinked polymers, particle shape, and the connectedness of open pores between particles, and surface modification of the swollen gel. Fluidity of the liquid passing through swollen particles differs depending on permeability of the superabsorbent polymer composition. A liquid cannot flow readily through a superabsorbent polymer composition with low permeability.
As one of the methods of increasing permeability of the superabsorbent polymer, there is a method of performing a surface crosslinking reaction after polymerization, in which silica or clay is added together with a surface crosslinking agent. For example, U.S. Pat. Nos. 5,140,076 and 4,734,478 disclose the addition of silica during surface crosslinking of dry superabsorbent polymer powders.
However, while permeability is improved by the addition of silica or clay, there are problems that water retention capacity or absorbency under load is reduced in proportion thereto, and separation from the superabsorbent polymer easily occurs by external physical impact during transport. Further, there has been no development of a superabsorbent polymer which shows a high absorption rate under load while maintaining liquid permeability, water retention capacity, and absorbency under load at a predetermined level or higher, thereby exhibiting a high absorption rate in practice when being applied to diapers, etc.